This invention relates generally to a pyroelectric detector device and in particularly to a pyroelectric detector amplifier circuit having a feedback path for multiplying the resistance of a detector element resistor load at non-zero frequencies.
Pyroelectric detectors produce small amounts of electric charge whenever their temperature is changed. Often, these detectors are designed to undergo changes in temperature in response to incident electromagnetic radiation. Thus, an electric charge can be generated in response to any situation where electromagnetic radiation can change the temperature of the detector. The charge can be measured by electronic instrumentation which conveys a result in generally usable form.
Pyroelectric detectors are not typically used to measure steady temperatures (as with a thermometer). With pyroelectric detectors, such measurement would require precise accounting for very small amounts of electric charge, which is not practical with available technology. Therefore, pyroelectric detectors are used to measure changes in temperature. A detector designed to respond to electromagnetic radiation thus produces electric charge in response to changes in radiation incident upon it.
A typical application of a pyroelectric detector is to detect human beings who emit infrared light (a form of electromagnetic radiation). Optical elements are designed to focus infrared light from a certain volume of space onto a pyroelectric detector. A human being moving into the volume of space will cause a change in the amount of electromagnetic radiation incident upon the detector, which in turn will produce a small amount of electric charge in response. By adding electric instrumentation to such a combination of optics and detectors, a human motion sensor is created.
Charge from a pyroelectric detector may be measured in many different ways. One common method is to connect a load resistor (RL) in parallel with the pyroelectric sensor element 2 positioned in a package adjacent to a window 1 as shown in FIG. 1, and to measure the transient voltage produced by the charge flowing from the pyroelectric sensor element 2 in response to a change in temperature. Due to the physical characteristics of the detector, the magnitude of the resultant voltage varies with the rate of temperature change. This variation is often described by means of a frequency response graph as shown in FIG. 2, which shows peak voltage per watt of incident energy (causing temperature change) versus the frequency of application of the energy.
When used in the parallel load resistor (RL) configuration, pyroelectric detectors exhibit a peak response at a certain frequency. The load resistor is one element determining the location of the low frequency shoulder of the peak. Increasing the amount of load resistance moves the peak to a lower frequency. For many applications, a low frequency peak is desirable. However, the necessary load resistor may be in the hundreds of gigaohms. Resistances of this magnitude are difficult to fabricate. Moreover, instrumentation circuits utilizing such resistances are often problematic.
Another conventional pyroelectric detector is shown in FIG. 3 comprising an operational amplifier 8 used as a buffer for the pyroelectric sensor element 7 located in a package adjacent to a window 6. The operational amplifier adds a much smaller offset voltage than does the FET in FIG. 1 and it duplicates the pyroelectric sensor element 7 output voltage within a factor closer to unity (0.9995 to 1.0). However, these attributes are typically not critical to pyroelectric detector applications, so an FET is the amplifier element most commonly used because it is less expensive.
U.S. Pat. No. 5,352,895, issued Oct. 4, 1994 to Masao Inoue discloses a pyroelectric device comprising a pyroelectric member for detecting infrared radiation, and a field effect transistor (FET) connected to the pyroelectric member for amplifying the relatively small amount of electric charge produced in response to incident energy. Capacitors are connected to the source and drain of the FET for stabilizing an applied voltage and to cut high-frequency induced noise. A gate resistor is connected between the terminals of the pyroelectric member. The pyroelectric member is made of PVD or PZT materials and the gate resistor used for fire detection has a resistance ranging from 5 to 50 gigaohms and in particular claimed for 10 gigaohms. However, the FET amplifier does not provide for multiplying the resistance of the gate resistor 12 at non-zero frequencies.
U.S. Pat. No. 5,262,647, issued Nov. 16, 1993 to Akira Kumada discloses an infrared detector with a pyroelectric detector element and an operational amplifier having a chopper control circuit that enables the detector to sense either a moving person or temperature. A chopper mechanism interrupts an infrared ray input to the pyroelectric infrared detector element. A gain control circuit is connected to the operational amplifier for controlling the amplifier gain. However, the operational amplifier does not provide for multiplying the resistance of the load resistor at non-zero frequencies.
Accordingly, it is therefore an object of this invention to provide a feedback buffer amplifier for receiving the output of a pyroelectric detector element at the junction of a load resistor (RL) for multiplying the resistance of the load resistor (RL) at non-zero frequencies.
It is another object of the invention to enable the use of smaller resistances, which are easier to fabricate, in the pyroelectric detector by providing an effective load resistance as a result of feedback from a buffer amplifier output which multiplies the resistance of the load resistor.
It is a further object of the invention to more easily manage a quiescent (zero-frequency) output of the pyroelectric detector with smaller load resistors, in the presence of a buffer amplifier input bias current.
These and other objects are further accomplished by a pyroelectric detector comprising means for sensing infrared radiation, means connected in parallel with the sensing means for providing a load to measure the sensing means output, means coupled to the output of the sensing means for buffering the output, and a feedback means connected from an output to an input of the buffering means for producing multiplication of the resistance of the load. The sensing means comprises a pyroelectric ceramic material. The load means comprises a first resistor in series with a second resistor. The feedback means comprises a capacitor connected between the buffering means output and the load means. The buffering means comprises a field effect transistor. The buffering means comprises an operational amplifier. The buffering means comprises a gain of near unity. The multiplication of the load resistance improves a peak low frequency response of the sensing means. The buffering means comprises a source resistor for providing a buffered output of the sensing means output from the buffering means.
The objects are further accomplished by a pyroelectric detector comprising means for sensing infrared radiation, means connected in parallel with the sensing means for providing a load to measure the sensing means output, the load comprising a first load resistor in series with a second resistor, means coupled to the output of the sensing means for buffering the sensing means output, the buffering means comprising an output divider which includes a first source resistor connected in series with a second source resistor, and the detector further comprising a feedback means connected from the junction of the first source resistor and the second source resistor to the junction of the first load resistor and the second resistor for producing multiplication of the load. The sensing means comprises a pyroelectric ceramic material. The feedback means of the buffering means comprises a capacitor. The buffering means comprises a field effect transistor. The buffering means comprises a gain of near unity. The multiplication of the load resistor improves a peak low frequency response of the sensing means.
The objects are further accomplished by a method of providing a pyroelectric detector comprising the steps of sensing infrared radiation, connecting a load in parallel with means for sensing the infrared radiation, buffering the output of the sensing means with amplifier means, and providing a feedback means in the amplifier means connected from an output to an input of the amplifier means for producing multiplication of the resistance of the load. The step of sensing infrared radiation comprises the step of using a sensor having pyroelectric ceramic material. The step of connecting the load comprises the step of connecting a first resistor in series with a second resistor. The step of providing the feedback means comprises the step of connecting a capacitor between the amplifier means output and the load at the amplifier means input. The step of buffering the output of the sensing means comprises the step of using a field effect transistor. The step of buffering the output of the sensing means comprises the step of using an operational amplifier. The step of providing feedback for producing multiplication of the resistance of the load improves a peak amplitude of a low frequency response of the sensing means.